1. Field of the Invention
The present invention relates in general to an apparatus for controlling an actual drive force to be applied to drive wheels of a motor vehicle, and more particularly to techniques for applying a braking torque to at least one drive wheel to control the actual drive forces of the drive wheels.
2. Discussion of the Related Art
Generally, such a wheel drive force control apparatus is provided for a motor vehicle which includes (a) a drive power source such as an engine and/or an electric motor, (b) at least one pair of drive wheels driven by the drive power source, (c) a differential gear unit disposed between the drive power source and each pair of drive wheels, for connecting the right and left drive wheels of the pair so as to permit the drive wheels to be rotated at different speeds and to equally distribute the driving torque of the drive power source to the drive wheels, so that a drive force is produced between each drive wheel and a road surface, and (d) a pair of brakes for braking the respective drive wheels, independently of each other.
Since the driving torque values distributed to the two drive wheels of each pair from the drive power source through the differential gear unit are always equal to each other, the sum of the actual drive forces of the two drive wheels is two times the actual drive force of the wheel whose critical drive force with respect to the road surface is smaller than that of the other drive wheel. The critical drive force is the maximum drive force that can be produced between the drive wheel and the road surface without slipping of the drive wheel on the road surface, while the differential action of the drive wheels by the differential gear unit is not limited. The critical drive force of each drive wheel depends upon a friction coefficient xcexc of the road surface and a vertical load acting on the drive wheel. The critical drive force of one of the two drive wheels of the pair may become smaller than that of the other drive wheel, due to a lower value of the friction coefficient xcexc of the road surface area on which the above-indicated one drive wheel is running, and/or a smaller value of the vertical load acting on the above-indicated one drive wheel, than the values of the other drive wheel. In this case, the above-indicated one drive wheel slips on the road surface, and the actual drive force of that drive wheel decreases, so that the driving torque distributed to the other drive wheel by the differential gear unit is reduced. As a result, the actual drive force of the above-indicated other drive wheel decreases, whereby the sum of the actual drive forces of the two drive wheels decreases, making it difficult to achieve smooth starting or acceleration of the vehicle. Namely, in the presence of a difference between the critical drive forces of the two drive wheels, the driving torque to be distributed by the differential gear unit to the drive wheel whose critical drive force is larger cannot be made large enough to assure the actual drive force of that drive wheel that is sufficiently close to its critical drive force, even while the critical drive force of that drive wheel is considerably larger than the actual drive force.
In the light of the above drawback where there exists a difference between the critical drive forces of the right and left drive wheels, the differential gear unit is conventionally provided with an exclusively designed limited slip differential (LSD) mechanism which is adapted to limit the differential action of the differential gear unit, for increasing the actual drive of the drive wheel whose critical drive force is larger, to a value close to the critical value.
However, the provision of such a LSD mechanism in the differential gear unit for each pair of drive wheels inevitably results in a considerable increase in the cost of manufacture and the weight of the differential gear unit.
It is therefore an object of the present invention to provide an apparatus for controlling actual drive forces to be applied to drive wheels of a motor vehicle, so as to maximize the actual drive forces, by utilizing a brake provided for each drive wheel and without using an exclusively designed limited slip differential mechanism.
The above object may be achieved according to a first aspect of the present invention, which provides an apparatus for controlling an actual value of a drive force produced between a road surface and each of a pair of drive wheels of a motor vehicle including (a) a drive power source for driving the pair of drive wheels, (b) a differential disposed between the drive power source and the pair of drive wheels and connecting the pair of drive wheels so as to permit a difference between rotating speeds of the drive wheels and to evenly distribute a driving torque of the drive power source to the pair of drive wheels, for thereby producing the drive force between the road surface and each drive wheel, and (c) a pair of brakes for braking the pair of drive wheels, respectively, independently of each other, the apparatus being characterized by comprising: a pair of wheel speed sensors for detecting the rotating speeds of the pair of drive wheels; and a brake control device responsive to output signals of the pair of wheel speed sensors, for activating one of the pair of brakes which corresponds to one of the drive wheels which has a smaller critical value of the drive force with respect to the road surface than the other of the drive wheels, to thereby apply a braking torque to the above-indicated one drive wheel for increasing an apparent value of the drive force of that one drive wheel, in order to increase the actual value of the other drive wheel which has a larger critical value of the drive force.
The critical drive force of one of the two drive wheels may become smaller than that of the other drive wheels, due to a lower value of the friction coefficient xcexc of the road surface area on which the above-indicated one drive wheel is running, and/or a smaller value of the vertical load acting on the above-indicated one drive wheel, than the values of the other drive wheel. The vertical load may be reduced upon vertical movement or jumping of the drive wheel away from the road surface. If the brake for the above-indicated one drive wheel having the smaller critical drive force value is activated in the above condition, a braking torque is applied to that one drive wheel in the same direction as the actual drive torque based on an actual friction force between the road surface and the drive wheel in question. Namely, the braking torque and the actual drive torque are simultaneously applied to that drive wheel in the same direction. As a result, the apparent value of the drive force of the drive wheel having the smaller critical drive force, as seen from the other drive wheel having the larger critical drive force, is increased, and the slipping of the drive wheel having the smaller drive force is restricted, whereby the speed difference between the two drive wheels is reduced. Accordingly, the drive torque to be applied to the drive wheel having the larger critical drive force is increased by an amount corresponding to the braking torque applied to the drive wheel having the smaller critical drive force, so that the actual drive force of the drive wheel having the larger critical drive force is increased toward the critical value.
The wheel drive force control apparatus according to the first aspect of this invention was developed based on the above finding. In the present apparatus, the brake control device is adapted to activate the appropriate brake for braking the drive wheel having the smaller critical drive force value, when the vehicle is running with the two drive wheels having the different critical drive force values with respect to the road surface. The application of a braking torque to the drive wheel having the smaller critical drive force results in an increase in the apparent value of the drive force of that drive wheel, thereby increasing the actual value of the drive force of the other drive wheel having the larger critical drive force value. Therefore, the brake control device provides substantially the same effect as provided where the apparatus is equipped with a limited slip differential mechanism.
Thus, the present wheel drive force control apparatus is capable of maximizing the actual drive force of each drive wheel, without using an exclusively designed limited slip differential mechanism.
The present first aspect of this invention is applied to a front-drive motor vehicle, a rear-drive motor vehicle, or a four-wheel-drive motor vehicle which will be described. The four-wheel-drive motor vehicle may be a full-time or a part-time four-wheel drive type.
Generally, the four-wheel-drive motor vehicle indicated above includes (a) drive power source for driving the four drive wheels, (b) a front differential connecting the front right and left drive wheels so as to permit these drive wheels to rotate at different speeds, (c) a rear differential connecting the rear right and left drive wheels so as to permit these drive wheels to rotate at different speeds, (d) a center differential connecting input shafts of the front and rear differentials so as to permit these input shafts to rotate at different speeds, and (e) four brakes for braking the respective four drive wheels, independently of each other. The center differential is adapted to distribute a driving torque of the drive power source to the input shafts of the front and rear differentials, and the front differential is adapted to evenly distribute a torque received by the input shaft thereof to the front right and left drive wheels, for thereby producing the drive force between the road surface and each of the front right and left wheels, while the rear differential is adapted to evenly distribute a torque received by the input shaft thereof to the rear right and left drive wheels, for thereby producing the drive force between the road surface and each of the rear right and left drive wheels.
In the four-wheel-drive motor vehicle, slipping of one of the four drive wheels will result in a decrease in the actual drive forces of all of the four drive wheels. To improve the starting and driving performance of the four-wheel-drive motor vehicle on a road surface having a relatively low friction coefficient xcexc or on a bad road surface, a differential lock mechanism is conventionally provided for at least one of the three differentials indicated above. Where the differential lock mechanism is provided for the center differential, the slipping of one of the four drive wheels will not cause a decrease in the actual drive forces of all of the four drive wheels. If, however, one of the front right and left drive wheels and one of the rear right and left wheels suffer from slipping, the drive forces of all of the four drive wheels decrease. If the differential lock mechanisms are provided for the center differential and one of the front and rear differentials, the actual drive forces of all of the four drive wheels will not decrease even if one of the front wheels and one of the rear wheels suffer from slipping. However, slipping of the three wheels of the four wheels will cause a decrease of the actual drive forces of all of the four wheels. If the differential lock mechanisms are provided for all of the center, front and rear differentials, the slipping of the three wheels will not cause a decrease in the actual drive forces of all of the four wheels.
Thus, the conventional four-wheel-drive motor vehicle has a limited slip differential mechanism in the form of a differential lock mechanism for at least one of the center, front and rear differentials, in order to improve the starting and driving performance of the vehicle on the low-friction-coefficient or bad road surface. The provision of the limited slip differential mechanism inevitably results in considerable increase in the cost of manufacture and the weight of the vehicle.
It is therefore another object of the present invention to provide an apparatus for controlling actual drive forces to be applied to drive wheels of a four-wheel-drive motor vehicle, so as to maximize the actual drive forces, by utilizing a brake provided for each drive wheel and without using an exclusively designed limited slip differential mechanism.
The above object may be achieved according to a second aspect of this invention, which provides an apparatus for controlling an actual value of a drive force produced between a road surface and each of four drive wheels consisting of a front right drive wheel, a front left drive wheel, a rear right drive wheel and a rear left drive wheel of a four-wheel drive motor vehicle including (a) a drive power source for driving the four drive wheels, (b) a front differential connecting the front right and left drive wheels so as to permit a difference between rotating speeds of the front right and left drive wheels, (c) a rear differential connecting the rear right and left drive wheels so as to a difference between rotating speeds of the rear right and left drive wheels, (d) a center differential connecting input shafts of the front and rear differentials so as to permit a difference between rotating speeds of the input shafts, and (e) four brakes for braking the four drive wheels, respectively, independently of each other, the center differential distributing a driving torque of the drive power source to the input shafts of the front and rear differentials, the front differential evenly distributing a torque received by the input shaft thereof to the front right and left drive wheels, for thereby producing the drive force between the road surface and each of the front right and left drive wheels, the rear differential evenly distributing a torque received by the input shaft thereof to the rear right and left drive wheels, for thereby producing the drive force between the road surface and each of the rear right and left drive wheels, the apparatus being characterized by comprising: four wheel speed sensors for detecting the rotating speeds of the four drive wheels, respectively; and a brake control device responsive to output signals of the four wheel speed sensors, and operable when the four drive wheels have a first and a second drive wheel having different critical values of the drive force with respect to the road surface, the critical value of the drive force of the first drive wheel being smaller than that of the second drive wheel, the brake control device activating one of the four brakes which corresponds to the first drive wheel, to thereby apply a braking torque to the first drive wheel for increasing an apparent value of the drive force of the first drive wheel, in order to increase the actual value of the second drive wheel.
The wheel drive force control apparatus constructed according to the second aspect of the invention described above is also capable of maximizing the actual drive force of each of the four drive wheels, without having to use a limited slip differential mechanism, for the same reasons as described above with respect to the first aspect of the invention.
In the present apparatus, the center differential may be arranged to either evenly or unevenly distribute the input drive torque to the front and rear differentials. Where the center differential is arranged for uneven distribution of the input drive torque to the front and rear differentials, the distribution ratio may be either fixed or variable.
According to a first preferred form of the apparatus of the second aspect of this invention, the brake control device includes control means operable when the four drive wheels includes a lowest-speed drive wheel whose rotating speed is the lowest of all of the four drive wheels. The control means is adapted to control three brakes of the four brakes which three brakes correspond to respective three drive wheels that are the four drive wheels except the lowest-speed drive wheel. The control means controls the three brakes, by using the lowest rotating speed of the lowest-speed drive wheel as a reference wheel speed, so as to substantially zero a difference of the rotating speed of each of the three drive wheels from the reference wheel speed.
In the above first preferred form of the apparatus, the lowest rotating speed of the lowest-speed drive wheel is used as the reference wheel speed for controlling the brakes for the other three drive wheels, such that the rotating speeds of these three drive wheels are controlled to approach the reference wheel speed. Accordingly, the difference of the rotating speeds of the front right and left wheels connected to each other by the front differential is reduced toward zero, and the difference of the rotating speeds of the rear right and left wheels connected to each other by the rear differential is also reduced toward zero. Further, the difference of the rotating speeds of the output shafts of the center differential (input shafts of the front and rear differentials) is also reduced toward zero. Thus, the present form of the apparatus provides substantially the same effect as provided by the limited slip differential mechanisms for all of the center, front and rear differentials.
Accordingly, the wheel drive force control apparatus according to the first preferred form of the second aspect of this invention assures improved starting and driving performance of the four-wheel-drive motor vehicle.
The wheel drive force control apparatus according to the first preferred form, which is arranged to improve the starting and driving performance of the motor vehicle as described above, does not permit the four drive wheels to rotate at different speeds, and therefore tends to suffer from a so-called xe2x80x9ctight corner braking phenomenonxe2x80x9d during turning of the vehicle. Further, this apparatus does not permit the vehicle operator to recognize a slipping or spinning action of a given drive wheel or wheels which may occur due to jumping thereof away from the road surface (e.g., bumpy road surface). In other words, the apparatus does not permit the vehicle operator to obtain sufficient information on the road surface condition.
It is therefore an optional object of the invention to permit some degree of difference of the rotating speeds of the four drive wheels, for thereby preventing the tight corner braking phenomenon and enabling the vehicle operator to obtain sufficient information on the road surface condition.
The above optional object may be achieved according to a second preferred form of the second aspect of this invention, wherein the brake control device includes control means for calculating a total speed control error for each of the front right and left drive wheels, and a total speed control error for each of the rear right and left drive wheels. The total speed control error for each front drive wheel consists of a first error component which is a difference obtained by subtracting a half of an average speed of the rear right and left drive wheels from a half of an average speed of the front right and left drive wheels, and a second error component which is a difference obtained by subtracting the average speed of the front right and left drive wheels from the speed of each front drive wheel. The total speed control error for each rear drive wheel consists of a first error component which is a difference obtained by subtracting a half of an average speed of the front right and left drive wheels from a half of an average speed of the rear right and left drive wheels, and a second error component which is a difference obtained by subtracting the average speed of the rear right and left drive wheels from the speed of each rear drive wheel. The control means is adapted to control each of the four brakes which corresponds to each of the four drive wheels whose total speed control error is a positive value, such that the positive total speed control error of each drive wheel is substantially zeroed.
In the wheel drive force control apparatus according to the second preferred form of the invention described above, the total speed control error for each of the front right and left drive wheels is obtained as the sum of the first error component and the second error component, as the control amounts suitable for controlling the front wheel brakes, so as to zero the speed difference of the center differential and the speed difference of the front differential. The first error component is obtained by subtracting a half of the average speed of the rear right and left wheels from a half of the average speed of the front right and left drive wheels, while the second error component is obtained by subtracting the average speed of the front drive wheels from the speed of each front drive wheel. On the other hand, the total speed control error for each of the rear right and left drive wheels is obtained as the sum of the first error component and the second error components, as the control amounts suitable for controlling the rear wheel brakes, so as to zero the speed difference of the center differential and the speed difference of the rear differential. The first error component for the rear drive wheels is obtained by subtracting a half of the average speed of the front drive wheels from a half of the average speed of the rear drive wheels, while the second error component for the rear drive wheels is obtained by subtracting the average speed of the rear drive wheels from the speed of each rear drive wheel. However, the rotating speeds of the drive wheels can be controlled by the brakes only where the total speed control error is a positive value. That is, the brakes can be used only when the speeds of the drive wheels are lowered, and cannot be used to raise the speeds of the drive wheels. Thus, the present form of the invention permits some degrees of difference between the speeds of the front and rear wheels, difference between the speeds of the front right and left wheels, and difference between the speeds of the rear right and left wheels. In other words, the four drive wheels are permitted to some extent to rotate at speeds different from each other.
Therefore, the apparatus according to the second preferred form of the invention described above prevents the tight corner braking phenomenon and enables the vehicle operator to obtain sufficient information on the road surface condition.
While the apparatus according to the first preferred form of the invention provides substantially the same effect as provided when the limited slip differential mechanisms are provided for the center, front and rear differentials, as described above, the apparatus according to the second preferred form provides substantially the same effect as provided when the limited slip differential mechanism is provided for only the center differential, but this effect is smaller than that provided when the limited slip differential mechanisms are provided for all of the three differentials.
According to a third preferred form of the second aspect of this invention, the brake control device includes control means for controlling two brakes of the four brakes which correspond to one of a first pair of wheel consisting of the front right and left wheels and a second pair of wheels consisting of the rear right and left drive wheels. The above-indicated one of the first and second pairs has a higher average speed than the other of the first and second pairs. In this case, the control means controls the two brakes, such that the average speed of the one of the first and second pairs is reduced toward the average speed of the other of the first and second pairs.
The wheel drive force control apparatus according to the third preferred form of the invention described above is adapted to control the brakes, by taking into account the speed difference between the front and rear drive wheels, namely, the speed difference of the center differential, but not taking into account the speed difference between the right and left drive wheels.
This third preferred form of the invention provides substantially the same effect as provided when the limited slip differential mechanism is provided for only the center differential.
According to a fourth preferred form of the second aspect of the invention, the brake control device includes at least two control means selected from the control means according to the first preferred form, the control means according to the second preferred form and the control means according to the third preferred form. In this case, the brake control device further includes selectively enabling means for selectively enabling one of the at least two control means to operate.
The wheeld drive force control apparatus according to the fourth preferred form described above provides an increased degree of freedom in controlling the brakes so as to control the drive forces of the drive wheels, than the apparatus according to the first, second or third preferred form, which includes only one of the three control means described above.
The selectively enabling means may include determining means for determining whether the motor vehicle is placed in a condition in which starting of the motor vehicle is difficult, and selecting means for selecting the control means according to the first preferred form if the determining means determines that the motor vehicle is placed in the above-indicated condition, and selecting one of the control means according to the second and third preferred forms if the determining means determines that the motor vehicle is not placed in the above-indicated condition. In this case, the selectively enabling means may include means for determining whether the rotating speed of any one of the four drive wheels is substantially zero, and determining means for determining whether an average rotating speed of the front right and left drive wheels is substantially equal to an average rotating speed of the rear right and left drive wheels. The determining means determines that the motor vehicle is placed in the above-indicated condition if the rotating speed of any one of the four drive wheels is substantially zero and if the average rotating speeds of the front right and left drive wheels and the rear right and left drive wheels are substantially equal to each other.
Alternatively, the selectively enabling means may include determining means for determining whether any one of at least one operator-controlled member is placed in a position for selecting the control means according to the first preferred form, and means for enabling the control means according to the first preferred form if the determining means determines that any one of the above-indicated at least one operator-controlled member is placed in said position.